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Title: Buoyant plumes in a turbulent environment
Author: Hübner, Jens
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2004
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This dissertation investigates how a nominally axisymmetric plume is affected by the presence of ambient turbulence. The results of this investigation are based on laboratory experiments in water. The experiments employ a saline plume released downwards into a tank of fresh water with an oscillating grid positioned near the bottom. A wide range of regimes with increasing levels of ambient turbulence is covered from a quiescent environment to the eventual break-up of the plume. Buoyancy and velocity data were obtained using fluorescent dye and particle tracking, respectively. The response of a plume to ambient turbulence is found to be twofold. The plume centreline position deviates from the perfect vertical and the plume spreading relative to that centreline position increases. Both mechanisms, plume "meandering" and "spreading", combine to create a time averaged plume image in which the plume appears wider. Sophisticated image processing techniques are developed to disentangle the two mechanisms and to quantify their respective contributions to the overall plume spreading. The turbulence-induced plume meandering scales on the ratio of the transversal velocity fluctuation of the background turbulence to the vertical mean velocity on the plume centreline. A simple extension to classical plume theory is proposed to account for the increased plume spreading via enhanced entrainment that becomes dependent on the strength of the ambient turbulence. Although in a quiescent environment the cross-sectional profiles of velocity and buoyancy are close to Gaussian in shape, the actual mechanism of plume spreading involves an inwards erosion of the Gaussian shape from the flanks of the profile by the ambient turbulence. Obviously, a simple extension to classical plume theory does not capture this qualitative change to the shape of the plume profiles. A scaling is proposed instead that links the radial extent to which the Gaussian profile is eroded by the ambient turbulence to the relative magnitudes of the fluctuating velocity in the ambient turbulence and on the centreline of a plume in a quiescent environment.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available